The Hubble Telescope: Star Birth Buy Astronomy Posters At AllPosters.com

If you look up at the sky on a dark, clear night, you can see nearly 2000 of the billions of stars in our galaxy - the Milky Way (a total of 8000 stars can be seen with the naked eye from Earth). Although they look like dots, they are really like our closest star - the Sun.

Like our Sun, some of them may have planets orbiting around them. But they are so far away we cannot directly see any planets. They are so far away that light - the fastest thing there is - takes 4 years to reach us from the nearest one after our Sun.

The Sun is a star. With the sole exception of the Sun, the stars appear to be fixed, maintaining the same pattern in the skies year after year. In fact the stars are in rapid motion, but their distances are so great that their relative changes in position become apparent only over the centuries.

A star is a large celestial hot, incandescent sphere held together by gravitational force, composed of hot gases (usually more than 90% hydrogen). The radiant energy is produced by fusion reactions, mainly the conversion of hydrogen into helium. As a result of these nuclear reactions the star emits electromagnetic radiation, especially light, and other forms of electromagnetic radiation.

The temperatures and luminosities of stars are determined by their masses. The most massive stars are about 100 times heavier than the Sun. The largest stars are red supergiants - they can be 1000 times bigger than the Sun. Large stars are often very luminous and hot, and so appear blue. The hottest stars are blue supergiants. Their surface temperature is five times hotter than the Sun's.

Medium-size stars, like the Sun, are yellow, while small stars are a dull red. The smallest stars are less than 1/20th of a solar mass. The smallest stars are neutron stars, only 10 miles across!

The oldest stars are 15 billion years old.

The universe contains billions of galaxies, and each galaxy contains billions of stars, which are frequently bunched together in star clusters of as many as 100,000. The stars visible to the unaided eye are all in our own galaxy, the milky way. The visible stars are divided into six classes according to their apparent magnitude. Stars differ widely in mass, size, temperature, age and luminosity. About 90% of all stars have masses between one tenth and 50 times that of the Sun. The most luminous stars (excluding supernovas) are about a million times more powerful than the Sun, while the least luminous are only a hundredth as powerful. Variable stars fluctuate in luminosity.

Red giants, the largest stars, are hundreds of times greater in size than the Sun. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius.

The central region, or core, has a temperature of millions of degrees. At this temperature nuclear energy is released by the fusion of hydrogen to form helium. By the time nuclear energy reaches the surface of the star, it has been largely converted into visible light with a spectrum characteristic of a very hot body.

The theory of stellar evolution states that a star must change as it consumes its hydrogen in the nuclear reactions that power it. When all its nuclear fuel is exhausted, the star dies, possibly in a supernova explosion.